Electronic structure of water from Koopmans-compliant functionals

TL;DR

This paper evaluates the electronic spectral properties of liquid water using Koopmans-compliant functionals, comparing different molecular dynamics approaches and analyzing how geometrical variations affect the band gap and density of states.

Contribution

It demonstrates that Koopmans-compliant functionals can efficiently approximate advanced GW methods and elucidates the impact of molecular geometries and nuclear quantum effects on water's electronic spectra.

Findings

O-H bond length is the primary factor affecting the band gap.

Different MD approaches cause up to 1 eV variation in the band gap.

Nuclear quantum effects broaden spectral peaks, improving experimental agreement.

Abstract

Obtaining a precise theoretical description of the spectral properties of liquid water poses challenges for both molecular dynamics (MD) and electronic structure methods. The lower computational cost of the Koopmans-compliant functionals with respect to Green's function methods allows the simulations of many MD trajectories, with a description close to the state-of-art quasi-particle self-consistent GW plus vertex corrections method (QSGW+f$_{xc}$). Thus, we explore water spectral properties when different MD approaches are used, ranging from classical MD to first-principles MD, and including nuclear quantum effects. We have observed that the different MD approaches lead to up to 1 eV change in the average band gap, thus, we focused on the band gap dependence with the geometrical properties of the system to explain such spread. We have evaluated the changes in the band gap due to variations in the intramolecular O-H bond distance, and HOH angle, as well as the intermolecular hydrogen bond O$\cdot\cdot\cdot$O distance, and the OHO angles. We have observed that the dominant contribution comes from the O-H bond length; the O$\cdot\cdot\cdot$O distance plays a secondary role, and the other geometrical properties do not significantly influence the gap. Furthermore, we analyze the electronic density of states (DOS), where the KIPZ functional shows a good agreement with the DOS obtained with state-of-art approaches employing quasi-particle self-consistent GW plus vertex corrections. The O-H bond length also significantly influences the DOS. When nuclear quantum effects are considered, a broadening of the peaks driven by the broader distribution of the O-H bond lengths is observed, leading to a closer agreement with the experimental photoemission spectra.